1. Field of the Invention
This invention relates to a valve - actuator coupling and, more specifically, to such a coupling which can limit the amount of force applied to the valve during backseating when the rod of the actuator acts on the valve stem to open the valve.
2. Description of the Prior Art
There have heretofore been utilized a number of means for opening and closing large valves in the feedwater and steam systems of nuclear power plants which include a backseating configuration, such as those disclosed in U.S. Pat. Nos. 2,663,318; 3,601,157 and 3,888,280. One such means which is frequently used is a valve actuator which incorporates a hydraulic cylinder and a gas accumulator, such as those disclosed in an application filed on the same date as the present application, entitled "Valve Actuator", by D. W. Duffey. In both the preferred embodiment and the prior art embodiment disclosed therein, hydraulic oil is directed to a piston within the hydraulic cylinder to open the valve in opposition to a precharged source of high pressure gas in an accumulator which gas acts on the other side of the piston. The high pressure gas maintained in the accumulator is of a sufficient quantity and pressure to act on the piston to rapidly close the valve when the hydraulic pressure is relieved. Under normal operating conditions, pressure limiting means, such as a hydraulic accumulator or a relief valve, on the hydraulic system which provides the hydraulic pressure for opening the valve is set at a level to properly backseat the valve when the piston is in an upward position. The resulting force acting on the stem, the difference between the hydraulic force upward and the gas force downward, is maintained to ensure proper backseating without damaging the valve. However, should a leak in or a rupture of the accumulator occur, the force generated by the hydraulic oil would no longer be opposed by the high pressure gas. Were this force to be transmitted to the backseat of the valve through the stem, permanent valve damage might occur. Although it is less likely to occur, the same detrimental results might be obtained with failure of the hydraulic pressure limiting means.
A concern for and a consideration of the amount of force applied to a valve during backseating has also affected the operation of other forms of valve actuators. Although they are not capable of closing this type of large valve as rapidly as can the accumulator described hereinabove, electric motor operators are frequently employed for valve operation in nuclear power plant systems. To close a valve with an electrical motor operator, a remote signal is sent to the motor to cause gear rotation in one direction to lower the rod. The motor gearing includes a torque limit switch which is contacted during a predetermined torque condition within the gearing so that when the rod has acted on the stem to fully seat the valve, motor power is discontinued by the torque limit switch. Motor operation is, therefore, automatically stopped when the valve is fully closed. It would seem that a similar torque limit switch could be employed in the motor gearing when the valve is opened. The motor would again stop when the valve is backseated as an increase in torque would again turn off the electrical motor. However, the level at which such a torque limit switch would be set to prevent damage during backseating would limit the torque throughout operation of the electrical motor operator while opening the valve. But it is often found that a significantly higher torque is needed simply to initially open the valve. A torque limit switch with such a setting would, therefore, prevent the valve from being opened by disconnecting the motor prior to any upward movement of the stem. Consequently, it is presently the practice in the operation of the electrical motor operator during opening of the valve to provide a position limit switch for stopping the motor just prior to backseating of the valve. A handwheel is provided for manually applying the proper torque to fully backseat the valve.
Last, but by no means least, there exists valve actuators which are operated by hydraulic oil which acts on both sides of a piston to position the valve. The use of hydraulic oil pressure to open the valve, without a controlled pressure on the other side of the piston in opposition thereto, has generally produced forces which are unsatisfactory for backseating the valve. Consequently, valves operated by a hydraulic actuator have not heretofore been provided with any simple, automatic means for backseating. A means for limiting the force which could be applied to the valve through the valve stem would, therefore, allow a simple backseating feature to be employed for the first time with this type of actuator.